In a conventional motor-driven mold clamping apparatus, mold clamping force is generated by use of a mechanism such that an output shaft of an electric motor such as a servomotor is connected to a toggle mechanism via a ball screw. Rotation generated through drive of the servomotor is transmitted via the output shaft to the ball screw and is then converted to thrust force by the balls crew. The thrust force is then amplified by the toggle mechanism to thereby obtain a mold clamping force. Further, a crosshead is disposed between the ball screw and the toggle mechanism, and the above-described thrust force is transmitted to the toggle mechanism via the crosshead.
In the conventional motor-driven mold clamping apparatus, in order to generate a preset mold clamping force, the crosshead must be moved to a precise position corresponding to the mold clamping force.
However, since the relationship between the position of the crosshead and mold clamping force becomes nonlinear as a result of the structural limitation of the toggle mechanism, calculation for obtaining the position of the crosshead is difficult. Therefore, a desired mold clamping force cannot be generated with precision. In addition, since each individual mold clamping apparatus involves mechanical error of a different degree, even if calculation for obtaining the position of the crosshead is possible, the calculation must be performed for each of the mold clamping apparatuses, resulting in an increase in cost thereof.
Moreover, when play is produced in movable parts such as the ball screw and pins and bushes of the toggle mechanism, or when the friction characteristics of the movable parts change, the mold clamping force changes accordingly, resulting in molded products of deteriorated quality.
An object of the present invention is to solve the above-mentioned problems in the conventional mold clamping apparatus, and to provide a mold clamping control apparatus capable of accurately generating mold clamping force, reducing the cost of the mold clamping apparatus, and improving the quality of molded products.